A Forbes ranking of San Diego as the sixth coolest city in the U.S., right behind San Francisco, can be true only if cool, at least as I grew up thinking about it, has turned a lot warmer.

When San Diego, Santa Ana and Riverside — Riverside! — make a Top 20 list of cool cities, has my old-school view of cool completely lost its cool?

Evidently so.

In the 1950s and ’60s, the generation born during the Depression and World War II schooled boomers on what it looked and felt like to be cool, a metaphor for ironic detachment from American conformity.

Embodied by black musicians like Miles Davis and white Pied Pipers like Jack Kerouac, James Dean and Bob Dylan, cool became a sort of watchword for outsider alienation.

Conspicuously cool San Diegans like Tom Waits and Frank Zappa left to make their edgy fortunes in the cooler clime of L.A.

In the seminal days of cool, two indisputably cool cities — New York and San Francisco — were the east and west poles. (Boston and L.A. chilled in the same magnetic field.)

In the ’60s, San Diego — KPRI radio station and Wahrenbrock’s bookstore aside — would not have been in most conversations about coolest cities.

In the Forbes methodology, a city’s youth, cultural diversity, net migration and non-chain restaurants figure in the cool metric that boosts San Diego.

What’s overlooked in the stats is the sturdy fiber of our region, which includes 17 other smaller cities.

For a century, San Diego has been a proud military partner. That used to be uncool in the old hipster anti-war sense, but it’s clearly good for the region’s economy and arguably its character.

Downtown San Diego has seen a surge of cool lofts and such, but more money has been poured into McMansions and gated communities, which is uncool by urban standards but fantastic, even cool, for families.

Our political leaders are, as a rule, conservatively uncool. If San Diego were cool, it might have gone rogue and elected the enigmatic Nathan Fletcher for mayor. Kevin Faulconer won because he’s genially uncool and a smooth fit in America’s Finest’s grain.

Cool? Maybe.

We have lots to love — museums, theaters, sports, a climate and topography to retire and die for — but are we cool?